When using MIMO channels, it is possible to transmit different data streams from different Tx antennas. This is generally known as a BLAST transmission. More specifically, in a MIMO system multiple parallel data streams are transmitted from different antennas. These data streams are fully interfering with each other, i.e., they are not separated by means of orthogonal spreading codes. A problem thus arises, as the terminal receiver must distinguish the data streams from one another.
During the use of the BLAST-type of MIMO transmission several sources of interference can exist. A first type of interference is the interference between parallel data streams, also referred to as inter-layer interference. A second type of interference is multiple access (intra-cell) interference, which typically results from multipath propagation. A third type of interference is inter-cell interference.
Conventional MIMO receivers may apply space or space-time equalization to suppress at least the first type of interference (inter-layer interference). However, since the interference suppression is imperfect the applicability of MIMO techniques in practical multipath channels is severely restricted.
As will be described below in further detail, several types of MIMO receiver architectures are currently known. A first type of MIMO receiver includes a space equalizer. However, the space equalizer performs spatial domain filtering only, and does not suppress multiple access interference. A second type of MIMO receiver includes a space-time equalizer. The space-time equalizer represents an improvement over the space equalizer, as it performs both spatial and time domain filtering, and also provides at least partial suppression of inter-layer and multiple access interferences. A third type of MIMO receiver operates by performing ordered successive interference cancellation. This type of MIMO receiver uses equalization and interference cancellation, and detects the data streams (i.e., the layers) in successive order. As each layer is detected it is then cancelled from the input signal. However, this type of MIMO receiver suffers from error propagation, and is furthermore highly complex to implement. A fourth type of MIMO receiver includes a Maximum likelihood (ML) detector, and operates with a despread symbol-level signal to search for a best symbol combination using ML criteria. However, the ML-based MIMO receiver does not suppress multiple access interference or inter-symbol interference due to multipath.
A model for a signal vector consisting of baseband signal samples around time instant i is defined to describe different MIMO receivers. A signal sample vector corresponding to receive antenna n is written as:
                                                        r              n                        ⁡                          (              i              )                                =                                    (                                                                                                                  r                        n                                            ⁡                                              (                                                  i                          -                                                      F                            1                                                                          )                                                                                                                                  ⋮                                                                                                                                      r                        n                                            ⁡                                              (                        i                        )                                                                                                                                  ⋮                                                                                                                                      r                        n                                            ⁡                                              (                                                  i                          +                                                      F                            2                                                                          )                                                                                                        )                        =                                                            ∑                                      m                    =                    1                                    M                                ⁢                                  (                                                                                    H                        mn                                            ⁡                                              (                        i                        )                                                              ⁢                                                                  d                        m                                            ⁡                                              (                        i                        )                                                                              )                                            +                                                n                  n                                ⁡                                  (                  i                  )                                                                    ,                            (        1        )            where dm(i) is an unknown symbol or chip vector from transmit antenna m, Hmn(i) is a channel matrix, the columns of which are the time-discrete channel impulse responses (from transmit antenna m to receive antenna n) each affecting its respective symbol or chip in vector dm(i). Vector nn(i) is the additive noise-plus-interference vector in receive antenna n (including inter-cell interference). Parameters F1 and F2 define the length of the signal vector. If both are set to 0, the signal consists only of one time instant.
It can be noticed that, due to layered MIMO transmission, M symbol streams are directly overlapping and interfering with each other (interlayer interference). Moreover, in multipath channels, channel matrix Hmn(i) is not diagonal, thereby causing symbols from a single transmit antenna to overlap. This overlap is the source of multiple access interference in CDMA based systems that apply orthogonal spreading codes. In TDMA-based systems the non-diagonality of the channel matrix causes inter-symbol interference.
The received signal vector in a receiver with N antennas can be obtained simply as
                              r          ⁡                      (            i            )                          =                              (                                                                                                      r                      1                                        ⁡                                          (                      i                      )                                                                                                                                                              r                      2                                        ⁡                                          (                      i                      )                                                                                                                    ⋮                                                                                                                        r                      N                                        ⁡                                          (                      i                      )                                                                                            )                    .                                    (        2        )            
FIG. 1 illustrates a conventional M-by-N MIMO antenna configuration with M Tx antennas 1 and N Rx antennas 2. In theory, an N-antenna array can cancel out N-1 interfering signals. Thus, an M-by-N MIMO configuration may, in theory, be implemented if N≧M, since each of the M data streams (layers) are interfered by M-1 data streams. A conventional technique to detect layer m is to combine antenna signals as:
                                                                        s                ^                            m                        ⁡                          (              i              )                                =                                    (                                                                                          w                                              m                        ,                        1                                            *                                                                                                  w                                              m                        ,                        2                                            *                                                                            …                                                                              w                                              m                        ,                        N                                            *                                                                                  )                        ⁢                          (                                                                                                                  r                        1                                            ⁡                                              (                        i                        )                                                                                                                                                                                r                        2                                            ⁡                                              (                        i                        )                                                                                                                                  ⋮                                                                                                                                      r                        N                                            ⁡                                              (                        i                        )                                                                                                        )                                      ,                            (        3        )            where symbol wm,n denotes the complex weighting factor applied to a signal sample from receive antenna n in order to detect transmitted symbol sm from transmit antenna m. (•)* and ^ denote complex conjugate and estimate, respectively.
In Eq. 3 a possible despreading operation is neglected for simplicity. It should be noted that to detect symbol interval i, only those signals received during that specific time interval need to be used. Thus, this detection method can be referred to as space-equalization, since the data streams are separated by using spatial processing only. However, the space-equalizer can suppress the interfering data streams completely only in noiseless situation. Especially in the presence of multipath channels the space-equalizer cannot separate the data streams properly. This is because all multipath propagated signals function as additional interfering signals, and the interference cancellation capability of an N-antenna array is exceeded.
In this case, space-time equalization can be applied by setting F1 and/or F2 greater than zero:
                                                                                                              s                    ^                                    m                                ⁡                                  (                  i                  )                                            =                                                                    w                    m                    H                                    ⁡                                      (                                                                                                                                                                                                                                                            r                                    1                                                                    ⁡                                                                      (                                                                          i                                      -                                                                              F                                        1                                                                                                              )                                                                                                                                                              …                                                                                                                                                                  r                                    1                                                                    ⁡                                                                      (                                                                          i                                      +                                                                              F                                        2                                                                                                              )                                                                                                                                                                                                                                    r                                    2                                                                    ⁡                                                                      (                                                                          i                                      -                                                                              F                                        1                                                                                                              )                                                                                                                                                              …                                                                                                                                                                                                                                                                                                                                                                          r                                    2                                                                    ⁡                                                                      (                                                                          i                                      +                                                                              F                                        2                                                                                                              )                                                                                                                                                              …                                                                                                                                                                  r                                    N                                                                    ⁡                                                                      (                                                                          i                                      -                                                                              F                                        1                                                                                                              )                                                                                                                                                              …                                                                                                                                                                  r                                    N                                                                    ⁡                                                                      (                                                                          i                                      +                                                                              F                                        2                                                                                                              )                                                                                                                                                                                                                                            )                                                  T                                                                                        =                                                w                  m                  H                                ⁢                                  r                  ⁡                                      (                    i                    )                                                                                                          (        4        )            
where (•)H denotes conjugate transpose. Vector wm is comprised of complex weighting factors used for combining signal samples from all N antennas from the specified time interval [i−F1, i+F2], as is applied for the detection of the mth data layer.
A goal of space-time equalization is to also remove the inter-symbol interference. The interference suppression is, however, not complete, since linear estimation is always a compromise between noise enhancement and interference suppression. That is, a zero-forcing equalizer would be strongly sub-optimal, since completely removing the interference results in an undesirable noise enhancement. Preferably, the linear minimum mean-square error (LMMSE) criterion is applied.
The space-time equalizer can also be used in combination with the ordered successive interference canceller. In this case the data layer having the strongest received signal is detected first by using an equalizer, and is then cancelled from the input signals. The remaining layers are detected similarly, assuming no residual interference from the already detected and cancelled layer(s). Preferably, pre-decoding interference cancellation is used, as using channel decoded data for interference cancellation requires complex processing operations (involving data buffering, re-encoding, re-spreading and cancellation steps).
MIMO reception can also be based on the ML principle, in which a “best” symbol combination is selected by analyzing through all possible symbol combinations.
In the case of wideband code division, multiple access (WCDMA) reception, there are typically several active users in the downlink signal. The users are separated by orthogonal Walsh spreading codes. A multipath channel, however, distorts the spreading codes and the user orthogonality is impaired. The non-zero cross-correlation between delayed versions of the spreading codes is a source of multiple-access interference, which can be addressed by using a form of channel equalization. However, the downlink signal is also scrambled by using a complex, pseudo-random scrambling sequence that effectively makes the users' spreading codes appear to be random in nature. This also makes the code cross-correlations random making it, in practice, impossible to remove the multiple-access interference from the despread, symbol-level signal. To overcome this problem, the channel equalization is preferably performed by using a chip-level signal, i.e., the signal before despreading and descrambling. When the received signal is despread after chip-level equalization, most of the multiple-access interference can be suppressed without any knowledge of the active interfering code channels.
In the WCDMA downlink that uses MIMO transmission there are thus two sources of interference: (i) interference between parallel data streams (the inter-layer interference referred to above); and (ii) interference between active code channels (the multiple access interference referred to above). Both forms of this interference can be suppressed by using a (chip-level) space-time equalizer, as shown in Eq. (4). The suppression is not, however, perfect, and this can severely limit the applicability of MIMO communication in practical (real-world) multipath environments. Indeed, many of the MIMO results published in the literature assume a single-path channel, in which case the simple space or spatial equalization approach alone would be sufficient.